1. Field of the Invention
The present invention relates generally to a method of assigning an optimal speech or transmission channel in a multi-station radio communications system, and more specifically to such a method wherein plural fixed stations share in common, all the channels assigned to a system. The present invention is able to minimize co-channel interference and is able to retain optimal channel assignment information in the case of a power failure which cuts off the power supply to one or more of the fixed station(s).
2. Description of the Prior Art
It is known in the art to apply a multi-station radio communications system wherein all of the channels assigned to the system are accessible by a plurality of fixed stations located in the system. This kind of channel assignment is referred to as dynamic channel assignment, and finds extensive application within relatively small service areas such as within a building, factory etc. On the contrary, an application of fixed channel assignment to each of the fixed stations is practically difficult or impossible due to the sophisticated nature of the propagation characteristics. The term "fixed channel assignment" implies that each of fixed stations closely located is assigned a plurality of fixed carrier frequencies. The fixed channel assignment features a so-called frequency reuse.
In order to avoid co-channel interference in either of dynamic or fixed channel assignment systems, it is well known in the art that a given fixed station selects an optimal channel according to channel assignment priority data which has been accumulated in the given station from the beginning of the system operation. More specifically, the channel assignment priority of a given channel at a given station is raised if it is accessed and used for communication, and is lowered if accessed but fails to establish communication. Viz., a channel which is more likely to provide optimal communication, is given a higher rating or priority over those which tends to exhibit less preferable characteristics and which are apt not to provide the required characteristics.
However, when the system is initially put into use, all of the channels provided for the system have the same priority. That is to say, there is no difference in the channel priority. According to one prior art technique, all of the fixed stations are set to select the lowest numbered channel at the initial stage of operation.
Accordingly, this prior art system has suffered from the drawbacks that as the fixed stations operate independently of one another and the lowest numbered channel is initially used at each of said mobile or stationary stations, in the event that relatively close stations are simultaneously contacted by different mobile units and the stations in question assign the same channel, situations wherein co-channel interference or cross-talk is highly apt to occur.
A further drawback has been encountered in that the channel assignment priority data which is compiled over a period of time is stored in a volatile memory which can be subject to undesirable erasure. More specifically, the above-mentioned prior art system is usually connected with a commercially available power supply, and hence in the event that a power failure or similar type of power disturbance occurs, the power supply which maintains the memory in question is temporarily disturbed/cut-off. In the event of such a memory loss, an undesirably long period of time is required before the channel status data is re-accumulated and the system is restored to the level of operation acquired prior the power interruption. Accordingly, the system is temporarily reduced to the situation wherein the initial cross-talk and/or co-channel interference problems are undesirably encountered all over again.